X-Hays and the Atomic Weight of Nickel. 421 



constituent electrons, factors we conceive o£ as governing the 

 atomic weight. 



We cannot, however, by any such physical methods take 

 the question beyond the region of extreme probability. But 

 it appears to us that these experiments furnish very strong- 

 evidence to support us in the conviction that the true atomic 

 weight of nickel is approximately mid-way between those of 

 cobalt and copper — very probably about 61*4. 



The principal points of interest in connexion with the 

 theory of secondary radiation brought out during these 

 experiments are the following : — 



The periodic relation between absorbability of the secondary 

 rays and the atomic weight of the radiator obtained by experi- 

 ments with aluminium as the absorber, is a true relation 

 between the general absorbability of the radiation emitted by 

 different atoms and their weight, for the variation in the 

 penetrating power of the secondary radiation with variation 

 in atomic weight of the radiator is practically the same with 

 all absorbing substances used, except in the case of special 

 relations existing between the radiator and absorber. 



The secondary <2?-rays emitted by certain substances are 

 remarkably homogeneous in character, though the primary 

 radiation producing them is very heterogeneous. This, how- 

 ever, is not true of the ^-radiations proceeding from all 

 elements even among those which emit a radiation differing 

 considerably from the primary in character. 



Change in the penetrating power of the primary beam has 

 not been found to affect the penetrating power of the 

 secondary from these substances, within certain limits in the 

 character of the primary. 



The fact that after transmission through plates of absorb- 

 ing substances the penetrating power of the secondary beams 

 from certain elements is not changed, shows that transmission 

 through substances does not modify the transmitted radiation 

 in any way. It has frequently been suggested that the 

 change observed in the penetrating power of ^-radiation after 

 transmission through absorbing substances is due to trans- 

 formation. We have found, however, that when these 

 apparently homogeneous beams are passed through any 

 substance not only is the general penetrating power un- 

 altered, but there is no special power of penetrating further 

 layers of the substance traversed. There appears then to be 

 little support for the theory that the special penetrating powers 

 are due to some specific property of the substance traversed, 

 other than that of exercising a selective absorption. It 

 should be noticed that what we have shosvn to be practically 



